1. Field of the Invention
The present invention relates to image processing. More specifically, the present invention relates to image enhancement through scene histogram modification.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Thermal imaging systems, such as Forward Looking Infrared (FLIR) systems, collect radiation from a scene to be imaged indicative of the temperature thereof. Infrared sensors typically convert this collected radiation to analog electrical signals which are passed through an analog to digital converter and stored in RAM memory. This digital data from the scene is processed and subsequently used to produce a static image or to feed video display driving circuitry.
The digital data from the scene is processed in order to control the intensity of the resultant static image or video display. For example, in the case of a conventional "black and white" video display the temperatures of the scene (as represented digitally in RAM) are mapped by an image processing scheme to a number of shades of gray between white and black. In some systems this mapping is linear. That is, the maximum and minimum digital RAM entries (corresponding to the maximum and minimum scene temperatures) are generally assigned to correspond to the image shades of white and black. A linear transfer function is then constructed between these temperature extremes and is used to map intermediate scene temperatures to corresponding shades of gray. Hence, in image processing utilizing linear mapping techniques the available shades of gray are allocated equally among the temperature ranges existing within the scene to be imaged.
FIG. 1a shows a histogram of the digital entries generated from thermal radiation received from a scene to be imaged. The horizontal axis of FIG. 1a corresponds to the range of digitally represented scene temperatures. The vertical axis of the histogram pertains to the number of memory locations containing a particular digital scene temperature. For example, digital scene temperature A is present in B memory locations. A linear transfer function L maps digital entries (scene temperatures) on the horizontal axis to a spectrum of the available shades of gray S. Inspection of FIG. 1a reveals that the temperatures of interest from the scene represented by the histogram of FIG. 1a are concentrated between the digital entries C and D and between the entries E and F. It follows that the shades of gray between points G and H and between points I and J were used in constructing an image of the scene. Unfortunately, as a result of the linear mapping function L the shades of gray between H and I were unused in generation of the scene image. Consequently, by allocating the available shades of gray uniformly throughout the temperature spectrum of the scene the linear mapping technique limits the image contrast obtainable within temperature ranges of interest.
FIG. 1b illustrates an image processing technique known as histogram equalization wherein the available shades of gray are allocated nonlinearly to the temperatures most prevalent within the scene to be imaged (see e.g., T. Pavlidis, Algorithms for Graphics and Image Processing). Specifically, the histograms of FIGS. 1a and 1b are derived from the same scene but in FIG. 1b the scene temperatures are mapped to a spectrum S' of the available shades of gray by a nonlinear mapping function N. The mapping function N is generated by integrating the histogram of FIG. 1b. As is evident upon inspection of FIG. 1b the spectrum of unused shades of gray is smaller in FIG. 1b than in FIG. 1a. Accordingly, a larger number of shades of gray are available to enhance image contrast within scene temperatures of interest by utilizing histogram equalization mapping rather than linear mapping.
Unfortunately, real-time video display systems are currently unable to directly implement histogram equalization because of computational constraints imposed by the available hardware. That is, the nonlinear mapping function N discussed above is not able to be modified quickly enough to adequately respond in real-time to a changing scene due to the large quantity of scene temperature data.
The problem of enhancing image detail (contrast) is addressed in certain imaging systems by including a relatively small "window" within the field of view of the scene. Detail within the window is to be enhanced by formulating a mapping function based exclusively on scene temperatures within this small window. For example, FIG. 2 shows the field of view F of a scene wherein a mapping function is constructed using scene temperatures from a window W included therein. The mapping function is generally constructed so as to maximize the number of shades of gray (contrast) available to the range of scene temperatures within the window W. In FIG. 2 the window W is chosen to include only the ground and not the sky so as to narrow the temperature range to which the available shades of gray are to be allocated. Further, real-time image processing is possible due to the reduced number of data points within the window W relative to the entire field of view F.
However, if as is shown in FIG. 3 the field of view F is translated such that the window W also includes a portion P of the sky, the scene temperature range from which the mapping function is constructed will again be comparable to that of the entire field of view F and little, if any, improvement in contrast will occur within the section of the resultant image corresponding to the window W. It follows that schemes to optimize contrast within a window of the field of view of a scene may be unsuitable in applications where real-time changes in the field of view can lead to a wide scene temperature differential.
Hence, a need in the art exists for a computationally efficient method, suitable for real-time applications, of controlling the intensity of an image of a scene.